Static async fn in traits

[tmandry]

This is a roadmap item for wg-async. You can view the roadmap here: wg-async roadmap

Top-line goal for wg-async in 2023. Shipping this requires some combination of:

### Research
- [x] #285 
- [x] #286 
- [ ] #287 
- [x] #288 
- [x] #300 
- [x] #302 

### RFCs
- [x] #289 
- [ ] #290 
- [ ] #291 
- [ ] #292, or maybe just a plan

### Implementation (TODO)